Seismic streamers are employed in marine exploration, and include a streamer cable towed behind a vessel and having spaced along the length thereof a plurality of cable leveling devices, commonly known as birds, which include controllable vanes by which the bird and cable are maintained at a predetermined depth. Seismic instrumentation usually disposed in pods along the streamer is operative to receive seismic reflections employed in a seismic or geologic survey being conducted. The control vanes are driven by on-board motors in the bird, and power for the motors and for vane control is provided by a battery source contained in the bird.
In one known control system, a signal wire extends through the seismic streamer cable which has one or more birds suspended from the streamer. A coil is disposed in the bird along an axis which is transverse to the nominal longitudinal axis of the signal wire running through the streamer. Signals transmitted along the wire are coupled by way of an air core inductive coupling including the coil in the bird and by which commands are coupled to the bird for control of the vanes. In another known system, two signal wires are provided in the streamer and a coil is connected across the signal wires and oriented along the longitudinal axis of the streamer at intended positions at which birds are disposed. Each bird includes a longitudinally oriented coil in coupling relationship with the coil within the streamer and by which signals transmitted along the streamer wires are coupled to the bird for vane control. The air core coupling relationship in both known systems is relatively inefficient, and in the latter system is subject to rather critical alignment of the transmitting and pick up coils to provide usable signal transfer.